1. Field of the Invention
This invention pertains to a neural or muscular tissue stimulating prosthesis capable of delivering a high current stimulation signal to a nerve, or brainstem, of a patient and, more particularly, to a cochlear prosthesis with a power supply having an output which can be selectively boosted to a high level.
2. Description of the Prior Art
Though the subject invention will find application with many types of tissue stimulating device it will be described in relation primarily to cochlear prosthesis systems. These prostheses are used to provide therapy to patients suffering from certain hearing impairing conditions. Frequently such systems are of a "two-part" design in that they comprise two sections: an internal or implanted section, and an external section. The external section includes a microphone for receiving ambient sounds and converting them to electrical signals. Power to the external section is provided by a battery. The electrical signals are processed and sent to the implanted section. The implanted section then generates excitation signals to excite the aural nerve of the patient by means of appropriately positioned stimulation electrodes.
Most commonly, the external section of a two-part cochlear prosthesis is inductively coupled by a transcutaneous RF link to the implanted section. The energy of the electrical signals in the RF frequency range is rectified and stored by a power supply located in the internal section. It is that power supply which provides the energy required to power the internal section and to generate the stimulus signals.
More recently there has been a trend in cochlear prosthesis design towards the use of totally implantable prostheses. In such devices the entire cochlear prosthesis, including a battery, is implanted. Obviously it is highly desirable that a totally implanted cochlear prosthesis be of as small a size as possible. In order to achieve the necessary miniaturisation it is important that the power supply, and so by necessity the battery, be of a small size.
To minimise the power requirements of the implanted section of a cochlear prosthesis, whether it be of the totally implanted or of the two-part type, it is desirable to operate it at as low a voltage as possible. One problem however with this approach is that a minimised voltage may present difficulties for the circuitry which is to apply the stimulation currents. In particular, a low operating voltage has hitherto reduced the maximum available amplitude of the stimulating signals that may be generated. An undesirable result is that the dynamic range of the stimulation signals conveyed to the patient is reduced so that loud sounds are perceived by the patient as being quieter than they should be.
Another problem, which is relevant only to cochlear prostheses of the two-part type, is that power supply voltage within the internal section is sensitive to the relative position and spacing of the coils used for the inductive coupling of the internal and external sections. When this positioning is not correct, the intercoil coupling is not optimal, and therefore the available power in the implanted section drops resulting in a limitation of the amplitude of the stimulation current that can be generated into the electrodes.
The problem of insufficient power being available to deliver the appropriate stimulations is especially acute for cochlear prostheses using biphasic stimulation current pulses. These pulses consist of two consecutive phases of opposite polarities with the first phase having a higher peak voltage amplitude than the second phase, due to the capacitive component of the load. If the power supply for the internal section has an inadequate voltage level (i.e., the power supply has a compliance problem), the current during the first phase of a pulse is smaller than required while the current during the second, lower voltage phase, remains unchanged thereby resulting in an unbalanced stimulation pulse.
In order to resolve these problems it has been proposed that, when sufficiently high voltage levels are not available, the duration of the biphasic pulse be increased to compensate, so that the charge delivered during each current phase remains approximately constant. However, the use of longer stimulation pulses inherently reduces the maximum stimulation rate of the device and so is undesirable.